Transition Metals

What is a transition metal?

A transition metal is a d-block element that forms at least one stable ion with an incomplete d sub-shell.

Why is Scandium not a transition metal?

Scandium only forms Sc³⁺, which has an empty d sub-shell (3d⁰).

Why is Zinc not a transition metal?

Zinc only forms Zn²⁺, which has a full d sub-shell (3d¹⁰).

List four typical properties of transition metals.

1. Variable oxidation states 2. Formation of coloured compounds 3. Catalytic activity 4. Formation of complex ions

What causes coloured compounds in transition metals?

The presence of partially filled d-orbitals allows d–d electron transitions when light is absorbed.

What is a complex ion?

A central metal ion surrounded by ligands, which are molecules or ions that donate lone pairs of electrons.

What is a ligand?

A molecule or ion that donates a lone pair of electrons to a metal ion to form a coordinate bond.

What is the coordination number of a complex ion?

The number of coordinate bonds formed with the central metal ion.

Give an example of a bidentate ligand.

Ethane-1,2-diamine (en) or ethanedioate (C₂O₄²⁻)

What is ligand substitution?

A reaction where one ligand in a complex ion is replaced by another.

What is the shape and bond angle of a complex with a coordination number of 6?

Octahedral, 90°

What is the shape and bond angle of a complex with a coordination number of 4?

Usually tetrahedral (109.5°) or square planar (90°)

What is a chelate?

A complex with multidentate ligands that form ring structures with the metal ion.

Why are chelates more stable than complexes with monodentate ligands?

Due to the chelate effect – increased entropy when multidentate ligands replace monodentate ones.

Why do transition metals have variable oxidation states?

The energies of the 4s and 3d orbitals are similar, allowing different numbers of electrons to be lost.

What colour is [Cu(H₂O)₆]²⁺ in solution?

Pale blue

What happens when aqueous ammonia is added to [Cu(H₂O)₆]²⁺?

A ligand substitution occurs forming [Cu(NH₃)₄(H₂O)₂]²⁺, deep blue solution.

What happens when NaOH is added to Fe²⁺ ions?

Green precipitate of Fe(OH)₂ forms, which darkens on standing due to oxidation to Fe(OH)₃.

How does vanadium act as a catalyst in the Contact Process?

V₂O₅ oxidises SO₂ to SO₃ and is reduced to V₂O₄, then reoxidised by O₂.

What is the role of Fe²⁺ in the reaction between S₂O₈²⁻ and I⁻?

It acts as a catalyst by transferring electrons between S₂O₈²⁻ and I⁻ via Fe³⁺/Fe²⁺ redox cycle. Give the colour of [Fe(H₂O)₆]²⁺ in solution.

What happens when NaOH is added to [Fe(H₂O)₆]²⁺?

Green precipitate of Fe(OH)₂ forms; it turns brown on standing due to oxidation to Fe(OH)₃.

Give the colour of [Fe(H₂O)₆]³⁺ in solution.

Yellow or violet (depending on concentration and hydration)

What happens when NaOH is added to [Fe(H₂O)₆]³⁺?

Brown precipitate of Fe(OH)₃ forms.

Give the colour of [Cu(H₂O)₆]²⁺ in solution.

Pale blue

What happens when excess NH₃ is added to [Cu(H₂O)₆]²⁺?

Deep blue solution of [Cu(NH₃)₄(H₂O)₂]²⁺ forms (ligand substitution).

What is observed when HCl is added to [Cu(H₂O)₆]²⁺?

Yellow solution forms due to formation of [CuCl₄]²⁻; may appear green due to mixture.

Give the colour of [Co(H₂O)₆]²⁺ in solution.

Pink

What happens when HCl is added to [Co(H₂O)₆]²⁺?

Blue solution of [CoCl₄]²⁻ forms (ligand substitution).

Write the overall redox reaction between Fe²⁺ and MnO₄⁻ in acidic solution.

MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O

What colour change occurs during the redox titration of Fe²⁺ with MnO₄⁻?

Purple MnO₄⁻ decolourises to colourless/pale pink as it is reduced to Mn²⁺.

How is Cr₂O₇²⁻ reduced to Cr³⁺ in acidic solution?

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

What is the colour change when Cr₂O₇²⁻ is reduced to Cr³⁺?

Orange to green

How is Cr³⁺ further reduced to Cr²⁺?

Using zinc metal in acidic solution

What is the colour of Cr²⁺?

Blue

How can Cr³⁺ be oxidised to CrO₄²⁻?

By warming with H₂O₂ in alkaline solution

What is the colour change when Cr³⁺ is oxidised to CrO₄²⁻?

Green to yellow

Write the reaction for the ligand substitution of [Cr(H₂O)₆]³⁺ with excess NH₃.

[Cr(H₂O)₆]³⁺ + 6NH₃ → [Cr(NH₃)₆]³⁺ + 6H₂O

What is the chelate effect?

The increased stability of complexes with multidentate ligands due to an increase in entropy.

Why does [Ni(en)₃]²⁺ form more readily than [Ni(NH₃)₆]²⁺?

Because it increases entropy – three bidentate ligands replace six monodentate ones (chelate effect).

What is autocatalysis?

A reaction where one of the products acts as a catalyst for the reaction itself.

Give an example of an autocatalysed reaction involving Mn²⁺.

The reaction between MnO₄⁻ and C₂O₄²⁻ is autocatalysed by Mn²⁺ ions.

What is the colour change when [Co(H₂O)₆]²⁺ is heated?

Pink to blue as [CoCl₄]²⁻ forms in the presence of HCl.